1. Field of the Invention
The present invention relates to a wet-type electrophotographic image formation method, and more particularly to a wet-type electrophotographic image formation method using a liquid developer comprising a carrier liquid comprising a silicone oil and toner particles dispersed therein, which liquid developer is particularly suitable for use with an organic photoconductor.
2. Discussion of Background
Generally in a wet-type electrophotographic image formation method, a latent electrostatic image formed on an electrophotographic photoconductor is developed to a visible toner image with a liquid developer comprising a carrier liquid and electrically charged toner particles dispersed therein. More specifically, in the wet-type electrophotographic image formation method, the latent electrostatic image is brought into contact with the liquid developer, and the electrically charged toner particles dispersed in the carrier liquid are deposited on the latent electrostatic image, so that the latent electrostatic image is developed to a visible toner image. The thus formed toner image on the photoconductor is transferred to a sheet of paper and then fixed on the paper with application of heat thereto.
Since the liquid developer employed in the above wet-type electrophotographic image formation method comprises finely-divided toner particles, generally having a particle diameter of 1 .mu.m or less, the wet-type electrophotographic image formation method has the advantage that the reproduction of fine line images, gradation and color images is excellent.
Furthermore, in the wet-type electrophotographic image formation method, toner particles are never scattered in the air, as occurs with a dry-type electrophotographic image formation method using a dry-type toner. Moreover, since the toner particles can be uniformly dispersed in the carrier liquid, they can be uniformly deposited on the latent electrostatic images formed on the photoconductor. This method is therefore adaptable to a high speed image formation process.
In comparison with organic photoconductors, however, the above inorganic photoconductors have the drawbacks that the cost is higher and they cannot easily be worked into a belt-type photoconductor because of their poorer flexibility. This will limit the incorporation or layout of the photoconductor in the copying apparatus. In addition to the above, the inorganic photoconductors show no photosensitivity in a long wave-length light region, so that a semiconductor laser beam cannot be used as a light source for forming light images. Accordingly, the manufacturing cost of a printer and a digital-type copying apparatus using the inorganic photoconductors is high and it is difficult to fabricate an apparatus which is compact in size.
The conventionally employed carrier liquids for the liquid developers for use in the wet-type electrophotographic image formation method are isoparaffin-based solvents, such as, for example, those commercially available under the trademark of "Isopar", made by Exxon Chemical Japan Ltd.
The carrier liquids of this kind can only be used with inorganic materials such as selenium, selenium-tellurium and arsenic selenium and are not suitable for use with an organic electrophotographic photoconductor which comprises an electroconductive support and an organic photoconductive layer formed thereon. This is because when the conventional isoparaffin-based carrier liquids contact the organic photoconductor for an extended period of time, a component which imparts the photosensitivity to the organic photoconductor contained in an organic photoconductive layer thereof is caused to ooze therefrom and flows into the liquid developer. Thus the photosensitivity of the organic electrophotographic photoconductor gradually deteriorates as the latent electrostatic images formed on the organic photoconductor are repeatedly developed with the liquid developer over a long period of time. In particular, when the organic photoconductive layer is of a function-separation type, which comprises a charge generation layer comprising a charge generating material, for example, the materials as disclosed in U.S. Pat. No. 4,150,987 and U.S. Pat. No. 4,391,889, and a charge transport layer comprising a charge transporting material and a binder resin, the charge transporting material is readily caused to ooze from the charge transport layer while in contact with the liquid developer and is mixed with the liquid developer. This will cause the photosensitivity of the photoconductor to deteriorate considerably.
In the case where the organic photoconductive layer is prepared by dispersing finely-divided particles of the charge generating material in a solid solution comprising a charge transporting material and a binder resin, the photosensitivity of the photoconductive layer also deteriorates while in contact with the liquid developer comprising an isoparaffin solvent.